Window blinds can be used to shade the interior of a passenger vehicle. A distinction between manually operated window blinds and electrically operated window blinds is that manual window blinds have a winding shaft rotatably seated in the window body underneath the lower edge of the window. One end of the blind material or sheet is fastened to the winding shaft. The other end of the blind sheet is remote from and extends parallel to the winding shaft and is fastened to a pull bar. The winding shaft is biased by a spring motor in the direction in which the blind sheet winds onto the winding shaft.
The pull bar has a handle that can be gripped manually. When pulling the blind out, the handle is gripped and moved towards the upper edge of the window. This pulls the blind sheet off the winding shaft against the force of the spring motor. In the uppermost position, the pull bar, handle or a special hook is hung on a fixture situated on the window frame in order to keep the window blind stretched out in front of the window.
To retract the window blind sheet, the handle again must be gripped in order to unhook the rod from the suspension device and guide it by hand to the lower edge of the window. The window blind sheet is simultaneously wound onto the winding shaft. The blind sheet runs through a slot between the window pane and parts of the inside trim of the vehicle door.
Manual operation of the blind sheet is not always compatible with modern electrically operated vehicle windows. While it is possible to open the window electrically when the blind sheet is extended, the blind sheet can begin to flutter because it is subjected to the air flowing past on the outside. In such a case, there is a substantial risk of damaging the window blind.
Once the window has been opened it is also not easy to wind the window blind sheet onto the winding shaft in a controlled manner. Depending on the pressure conditions around the vehicle body, the air flowing past the vehicle can tend to either draw the window blind sheet to the outside of the vehicle or to push it into the vehicle's interior. When the blind sheet is pushed into the interior of the vehicle it can lead to a considerable balloon effect that can make retraction of the window blind sheet difficult. In particular, the force created by the air can be greater than the force that the spring motor can exert causing a large balloon to be created as the pull bar is guided downwards, without any of the window blind sheet being wound onto the winding shaft.
In addition to these known manual window blinds, an electrically operated window blind is disclosed in DE 100 05 970 A1. Two guides are provided in the body of the vehicle door in which flexible support rods run perpendicular to the lower edge of the window. These support rods serve to stretch out a blind sheet which is connected to a winding shaft. The winding shaft is, in turn, rotatably seated underneath the lower edge of the window and biased by an electric motor in the direction in which the blind sheet is wound up on the winding shaft. The two support rods, which move the blind sheet while guiding it in a direction perpendicular to the window pane, act on the free edge of the blind sheet, i.e., the edge that travels the furthest in the stretching-open direction. In this manner, lateral guide rails are not required. It is therefore particularly suitable for those vehicle doors in which the window has no lateral and upper frame, as is common for coupes, for instance.
The two support rods are driven by linear flexible drive members that have teeth around their outer surface. The drive members mesh with a gearwheel seated on the output shaft of a geared motor. The support rods are advanced or retracted by setting the geared motor in motion. When advancing, the support rods carry the blind sheet and tension it in front of the window pane against the force of the spring motor. If the drive motor is operated in the opposite direction, the support rods are retracted and the spring motor is allowed to wind the blind sheet onto the winding shaft.
The cost of such an automatically operated window blind is relatively high compared to a manual window blind because an additional electric motor is required. Moreover, additional wiring and an additional operating switch are also required in the vehicle further adding to the expense.